A bipolar power semiconductor device such as, for example, a power diode, a power IGBT, or a power thyristor, includes a first emitter region of a first conductivity type (doping type), a second emitter region of a second conductivity type, and a base region (often referred to as drift region) of the first conductivity type. Usually, the base region has a lower doping concentration than each of the first and second emitter regions.
A bipolar power semiconductor device can be operated in two different operation states, namely a conducting state (on-state), and a blocking state (off-state). In the conducting state, the first emitter region injects charge carriers of the first conductivity type into the base region, and the second emitter region injects charge carriers of the second conductivity type into the base region. These charge carriers injected into the base region by the first and second emitters form a charge carrier plasma in the base region.
When the bipolar power semiconductor device switches from the conducting state into the blocking state these charge carriers are removed from the base region. Losses that occur in the transition phase from the conducting state to the blocking state are dependent on how many charge carriers are present in the base region before the semiconductor device starts to switch from the conducting state to the blocking state, whereas the higher the amount of charge carriers the higher the losses. Basically, the number of charge carriers can be adjusted by adjusting the charge carrier lifetime, in particular the minority charge carrier lifetime, which is the average time it takes for a minority charge carrier to recombine. The shorter the minority charge carrier lifetime, that is the faster minority charge carriers recombine, the lower is the amount of charge carriers in the base region at the time of switching from the conducting state to the blocking state. However, conduction losses, which are losses that occur in the bipolar power semiconductor device in the conducting state, increase as the charge carrier lifetime decreases.
When the bipolar power semiconductor device switches from the conducting state to the blocking state a depletion region expands in the base region beginning at a pn junction between the base region and the second emitter region. Through this, charge carriers forming the charge carrier plasma are removed from the base region; this is known as reverse recovery. During reverse recovery there is a reverse recovery current flowing between the first and second emitter region. Such reverse recovery current is caused by the removal of charge carriers from the base region. This current finally drops to zero as the charge carriers have been removed or recombined. A slope of this reverse recovery current as it tends to zero defines the softness of the component. The steeper the slope, the less “soft” is the reverse recovery behaviour (switching behaviour) of the semiconductor device. However, a soft behaviour is desirable, because steep slopes may cause voltage overshoots in parasitic inductances connected to the semiconductor device and/or may cause oscillations or ringing in a circuit in which the semiconductor device is employed.
A soft reverse recovery behaviour can be obtained by having a “charge carrier reservoir” in those regions of the base region that are depleted towards the end of the switching process, wherein this charge carrier reservoir feeds the reverse recovery current towards the end of the switching process so as to soften a decrease of the reverse recovery current to zero. Such a “charge carrier reservoir” can be obtained by having a high charge carrier lifetime in those regions of the base region that are depleted towards the end of the reverse recovery process.
There is therefore a need to suitably adjust the charge carrier lifetime in a bipolar semiconductor device in order to have low switching losses and a soft switching behaviour.